US20110205550A1 - Optical tomographic imaging method and apparatus - Google Patents
Optical tomographic imaging method and apparatus Download PDFInfo
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- G01B9/00—Measuring instruments characterised by the use of optical techniques
- G01B9/02—Interferometers
- G01B9/02055—Reduction or prevention of errors; Testing; Calibration
- G01B9/02062—Active error reduction, i.e. varying with time
- G01B9/02063—Active error reduction, i.e. varying with time by particular alignment of focus position, e.g. dynamic focussing in optical coherence tomography
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- A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
- A61B3/102—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for optical coherence tomography [OCT]
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- G01B9/00—Measuring instruments characterised by the use of optical techniques
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- G01B9/02083—Interferometers characterised by particular signal processing and presentation
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- G01B9/00—Measuring instruments characterised by the use of optical techniques
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Abstract
An optical tomographic imaging method is provided in which light from a light source is split into a measuring beam and a reference beam, the measuring beam being moved by a scanning optical system and guided to an object to be examined, the reference beam being guided to a reference mirror, and in which a tomogram of the object is generated from a return beam of the measuring beam reflected or scattered by the object and the reference beam reflected by the reference mirror. The method includes acquiring longitudinal sectional information on the object, calculating depth-position information on the object from the longitudinal sectional information, and acquiring a three-dimensional surface image of the object by controlling a reference-path length defined by the reference mirror and a scanning operation of the scanning optical system in accordance with the depth-position information on the object.
Description
- This application is a Continuation of U.S. patent application Ser. No. 12/636,607, filed Dec. 11, 2009, entitled “OPTICAL TOMOGRAPHIC IMAGING METHOD AND APPARATUS”, the content of which is expressly incorporated by reference herein in its entirety. This application also claims priority from Japanese Patent Application Nos. 2008-331879, filed Dec. 26, 2008, which is hereby incorporated by reference herein in its entirety.
- 1. Field of the Invention
- The present invention relates to optical tomographic imaging methods and apparatuses, and in particular to an optical tomographic imaging method and apparatus employing optical coherence tomography in the fields of ophthalmic diagnosis and the like.
- 2. Description of the Related Art
- In recent years, imaging apparatuses utilizing interference of low-coherence light have been in practical use.
- In particular, an optical tomographic imaging apparatus based on optical coherence tomography (hereinafter abbreviated as OCT) in which multiple-wavelength interference is utilized provides a high-resolution tomogram of a sample. OCT apparatuses have been becoming essential, particularly in the field of ophthalmic diagnosis, for acquisition of a tomogram of the eyeground or a region therearound.
- Other than in ophthalmic diagnosis, OCT apparatuses are also used in, for example, tomographic examination of the skin and, by being incorporated into endoscopes and catheters, in tomography of the walls of digestive and circulatory organs.
- Such OCT methods include a time-domain OCT (TD-OCT) method. Examples are disclosed in U.S. Pat. No. 5,321,501 and PCT Japanese Translation Patent Publication No. 2002-515593 (pages 9 to 19 and FIGS. 1 and 2 therein).
- A TD-OCT method will now be described.
-
FIG. 2 is a diagram of a known TD-OCT apparatus. - Referring to
FIG. 2 , light emitted from alight source 2101 travels through anoptical fiber 2102, is guided to acollimator 2103, and is output as a collimatedbeam 2104 from thecollimator 2103. - The collimated
beam 2104 is split into areference beam 2106 and asignal beam 2107 by abeam splitter 2105. - The
reference beam 2106 is reflected by a movable reference mirror 2111-2. - The movable reference mirror 2111-2 is moved by a
reference mirror stage 2112 mechanically movable in a one-dimensional direction, thereby controlling the measurement point in anobject 2117 to be examined. The measurement point is controlled in an optical-axis direction of thesignal beam 2107 entering theobject 2117. - The
signal beam 2107 is directed by a scanningoptical system 2108 in such a direction as to travel through eyepiece members 2109-1 to 2109-3, thereby entering theobject 2117. - The beam that has entered the
object 2117 is reflected by layers in theobject 2117 and returns to thebeam splitter 2105. - The scanning
optical system 2108 performs scanning by moving thesignal beam 2107 entering theobject 2117 in specific directions. - The reflections from the movable reference mirror 2111-2 and the
object 2117 enter thebeam splitter 2105 and interfere with each other, producing aninterference beam 2113. Theinterference beam 2113 is condensed by acollimator 2114 and is guided to adetector 2115. - The condensed beam is detected as a signal by the
detector 2115 and is imaged by acontrol computer 2116. - The
control computer 2116 controls the eyepiece member 2109-2 by moving a focusingstage 2110 so as to adjust the focus of the beam incident on theobject 2117. - The
control computer 2116 also controls thereference mirror stage 2112, thereby being capable of identifying, with reference to the detected interference signal and the position of thereference mirror stage 2112, the depth in theobject 2117 indicated by the detected signal. - Thus, in the TD-OCT method, image data can be generated from pieces of data representing the intensity of the interference beam, the pieces of data being successively acquired by performing scanning with the scanning
optical system 2108 while controlling the movable reference mirror 2111-2. - For example, in A-scan (scanning in the optical-axis direction of a beam entering an object, i.e., the depth direction of an object), the
signal beam 2107 entering theobject 2117 is moved by the scanningoptical system 2108 in one direction (for example, the X-axis direction) within a plane in theobject 2117. - Thus, pieces of one-dimensional A-scan data can be successively acquired.
- From such successively acquired images, a B-scan image (a two-dimensional image of a longitudinal section) can be acquired.
- If the
signal beam 2107 is moved in two directions (for example, the X- and Y-axis directions) within the foregoing plane without moving thereference mirror stage 2112, a C-scan image (a two-dimensional image of a transverse section) can be acquired. - If the
signal beam 2107 is moved in two directions (for example, the X- and Y-axis directions) within the foregoing plane while thereference mirror stage 2112 is controlled, a three-dimensional image can be acquired. - Here, A-scan, B-scan, and C-scan mentioned above will be described with reference to
FIG. 3 . - A
signal beam 3207 is made to enter anobject 3217 to be examined, as shown inFIG. 3 . - The
signal beam 3207 is made to enter theobject 3217 in the Z-axis direction inFIG. 3 . Therefore, in A-scan, information on a structure extending along anaxis 3218 in theobject 3217 is acquired. - If, in
FIG. 3 , thesignal beam 3207 is moved in the X-axis direction while scanning in the Z-axis direction is performed, information on aplane 3219 is acquired. - The
plane 3219 is a longitudinal sectional image. A scanning method providing such an image is referred to as B-scan. - If scanning is performed in the X- and Y-axis directions but not in the Z-axis direction, information on a
plane 3220 is acquired. - The
plane 3220 is a transverse image of a layer in theobject 3217. A scanning method providing such an image is referred to as C-scan. - There is another OCT method, a spectral-domain OCT (SD-OCT) method. An example is disclosed in “Handbook of Optical Coherence Tomography” (2006; FIGS. 2 and 3 in pages 145 and 149, and FIG. 1 in page 338).
- The SD-OCT method will now be described.
-
FIG. 4 is a diagram of a known SD-OCT apparatus. - The SD-OCT apparatus shown in
FIG. 4 differs from the TD-OCT apparatus shown inFIG. 2 in the following aspects. - The SD-OCT apparatus includes a
fixed reference mirror 4323 instead of the movable reference mirror 2111-2, spectroscope members 4321-1 and 4321-2 including a diffraction grating, and aspectroscopic detector 4322, such as a line sensor, instead of thedetector 2115. - In the SD-OCT apparatus, a spectrum acquired by the spectroscope members 4321-1 and 4321 is detected by the
spectroscopic detector 4322. - In the SD-OCT apparatus, the detected spectrum, which is composed of pieces of information on the intensity of an
interference beam 4313 expressed with respect to an axis representing the wavelength, is subjected to a Fourier transform into information expressed with respect to an axis representing the position of the scanned plane, whereby image data is acquired collectively in terms of time. - In the SD-OCT apparatus, since image data in the depth direction of an
object 4307 is collectively acquired, measurement speed can be increased compared with the case in the TD-OCT apparatus in which scanning in the depth direction is performed successively in terms of time. - It is known that SD-OCT apparatuses can acquire B-scan images and three-dimensional images at higher speeds than in TD-OCT apparatuses. Instead, it is pointed out that there is a difficulty in increasing the transverse resolution in SD-OCT apparatuses.
- This is because, in SD-OCT apparatuses, the measurement range of an object in the depth direction thereof is inversely proportional to the transverse resolution. Therefore, to acquire information on a longitudinal section having a sufficient area, the transverse resolution needs to be reduced.
- In recent years, there has been a demand for OCT apparatuses having increased image resolutions.
- There has also been a strong demand for capability of concentrated examination of an arbitrary layer in an object, and accordingly a demand for apparatuses capable of taking C-scan images with high resolutions and at high speeds.
- The known TD-OCT and SD-OCT apparatuses described above, however, have the following problems in satisfying such recent demands.
- For example, the known TD-OCT apparatus described above can acquire a two-dimensional image of a transverse section by C-scan in which the
signal beam 2107 is moved in two directions (for example, the X- and Y-axis directions) within a plane without moving thereference mirror stage 2112. - In this operation of the TD-OCT apparatus, only a signal from a specific depth defined by the movable reference mirror 2111-2 is acquired. Therefore, by focusing the
signal beam 2107 on the specific depth, a transverse-sectional image with a very high resolution can be acquired. - In this case, however, since the
signal beam 2107 is moved with the movable reference mirror 2111-2 fixed, only a two-dimensional image of a transverse-sectional image at a constant depth is acquired. - Therefore, in a case where a desired object to be examined is a curved layer, such as an internal structure of a retina, only fragmental transverse-sectional images of the desired layer are acquired.
- Another method can be considered in which three-dimensional information is generated from successively acquired B-scan images, and a surface of an arbitrary desired layer is extracted from the three-dimensional information. Such a method, however, requires a long time for acquisition of three-dimensional information. Besides, if the object moves during acquisition of such information, the resulting image may blur and clear information may not be acquired.
- In addition, to generate three-dimensional image information, high-load processing needs to be performed. This leads to a problem of a very long processing time.
- On the other hand, in the SD-OCT apparatus, information in the depth direction over a wide range can be acquired collectively, whereas it is difficult to increase transverse resolution.
- If the transverse resolution of the SD-OCT apparatus is increased, the longitudinal dimension of a range in which information can be collectively acquired is reduced, and the speed of information acquisition is also reduced to a level equal to or less than that in the TD-OCT apparatus.
- Moreover, to acquire information on an arbitrary surface, three-dimensional information first needs to be generated from a plurality of B-scan images, and then an image of the surface needs to be extracted, as in the TD-OCT apparatus.
- Such acquisition of high-resolution three-dimensional information also requires a very long time. Besides, if the object to be examined moves, the resulting image may blur.
- In addition, it takes a very long processing time, as in the TD-OCT apparatus.
- In light of the above, the present invention provides an optical tomographic imaging method and apparatus enabling acquisition of a surface image of an arbitrary layer in an object to be examined and acquisition of such a surface image in a short time even if the layer is curved.
- According to a first aspect of the present invention, an optical tomographic imaging method is provided in which light from a light source is split into a measuring beam and a reference beam, the measuring beam being moved by a scanning optical system and guided to an object to be examined, the reference beam being guided to a reference mirror, and in which a tomogram of the object is generated from a return beam of the measuring beam reflected or scattered by the object and the reference beam reflected by the reference mirror. The method includes acquiring longitudinal sectional information on the object, calculating depth-position information on the object from the longitudinal sectional information, and acquiring a three-dimensional surface image of the object by controlling a reference-path length defined by the reference mirror and a scanning operation of the scanning optical system in accordance with the depth-position information on the object.
- According to a second aspect of the present invention, an optical tomographic imaging apparatus is provided in which light from a light source is split into a measuring beam and a reference beam, the measuring beam being guided to an object to be examined, the reference beam being guided to a reference mirror, and in which a tomogram of the object is generated from a return beam of the measuring beam reflected or scattered by the object and the reference beam reflected by the reference mirror. The apparatus includes a scanning optical system capable of moving the measuring beam in one-dimensional and two-dimensional manners, a reference-path-length adjuster provided to the reference mirror and configured to adjust an optical-path length of the reference beam, and a control unit configured to control the reference-path-length adjuster and the scanning optical system so that a three-dimensional surface image of the object is acquired in accordance with depth-position information on the object estimated in advance.
- Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
-
FIG. 1 is a diagram for describing a TD-OCT apparatus and steps of a tomographic imaging method according to a first embodiment of the present invention. -
FIG. 2 is a diagram of a known TD-OCT apparatus. -
FIG. 3 is a diagram for describing A-scan, B-scan, and C-scan performed in the known TD-OCT apparatus. -
FIG. 4 is a diagram of a known SD-OCT apparatus. -
FIGS. 5A and 5B are diagrams for describing the optical path of an exemplary TD-OCT apparatus according to the first embodiment of the present invention in a case where the apparatus is used for eye measurement. -
FIGS. 6A to 6C are diagrams for describing steps of estimating a three-dimensional surface according to the first embodiment of the present invention. -
FIG. 7 is a diagram of an exemplary reference-path-length adjuster included in the TD-OCT apparatus according to the first embodiment of the present invention. -
FIG. 8 is a diagram for describing a method of controlling the position of the entrance pupil, instead of controlling a reference mirror, in controlling a plane along which information is to be acquired in the first embodiment of the present invention. -
FIG. 9 is a diagram for describing a TD-OCT apparatus and steps of a tomographic imaging method in Example 1 of the present invention. -
FIGS. 10A to 10C are diagrams for describing steps of estimating a three-dimensional surface in Example 1 of the present invention. -
FIGS. 11A to 11C are diagrams for describing steps of estimating a three-dimensional surface in Example 2 of the present invention. -
FIGS. 12A to 12C are diagrams for describing steps of estimating a three-dimensional surface in Example 3 of the present invention. -
FIG. 13 is a diagram for describing a TD-OCT apparatus and steps of a tomographic imaging method in Example 4 of the present invention. -
FIGS. 14A to 14C are diagrams for describing steps of estimating a three-dimensional surface in Example 5 of the present invention. - Exemplary optical tomographic imaging methods and apparatuses according to embodiments of the present invention will now be described.
- While the present invention is suitably applied to the field of tomographic imaging of the retina of an eye, the present invention is not limited thereto.
- For example, the present invention may also be applied to biomedical fields of endoscopy, skin examination, and the like, and to industrial fields of product quality control, product diagnosis, product inspection, and the like.
- A first embodiment of the present invention will now be described. The first embodiment concerns a TD-OCT-based optical tomographic imaging method and apparatus.
- According to a first exemplary embodiment of the present invention, an optical tomographic imaging method includes the following steps (1) to (4): (1) acquiring, as a first preliminary image, a two-dimensional image of an object to be examined, the image extending in an optical-axis direction in which light enters the object; (2) acquiring, as a second preliminary image, another two-dimensional image of the object, the image extending in the optical-axis direction and being substantially orthogonal to the two-dimensional image acquired in step (1); (3) calculating depth-position information on a desired layer from the image acquired in step (1) and the image acquired in step (2); and (4) acquiring a surface image of the desired layer by moving the light entering the object while controlling a reference mirror in accordance with the depth-position information.
- An optical tomographic imaging apparatus and details of the above steps according to the first embodiment will now be described.
-
FIG. 1 is a diagram for describing a TD-OCT apparatus and the steps of the tomographic imaging method according to the first embodiment. -
FIG. 1 shows alight source 101, anoptical fiber 102, acollimator 103, acollimated beam 104, abeam splitter 105, areference beam 106, asignal beam 107, a scanningoptical system 108, eyepiece members 109-1 and 109-2, a focusingstage 110, reference mirrors 111-1 and 111-2, areference mirror stage 112, aninterference beam 113, acollimator 114, aphotodetector 115, anOCT processing unit 116, and an object (eye) 124 to be examined. - In the TD-OCT apparatus of the first embodiment, light from the
light source 101 is split into thesignal beam 107, i.e., a measuring beam, and thereference beam 106, thesignal beam 107 being moved by the scanningoptical system 108 and guided to theobject 124, thereference beam 106 being guided to the reference mirrors 111. - A tomogram of the
object 124 is generated from a return beam of thesignal beam 107 reflected or scattered by theobject 124 and thereference beam 106 reflected by the reference mirrors 111. - The
light source 101 is, for example, a low-coherence light source or a superluminescent diode (SLD). The wavelength of thelight source 101 is not particularly limited to but is within 400 nm to 2 μm. For imaging of the eyeground, wavelengths of about 800 to 1500 nm are suitable. - The wavelength width for realizing OCT can be set to, for example, 1 pm or larger, specifically, 10 pm or larger, or more specifically, 30 pm or larger.
- The
light source 101 may alternatively be an ultrashort-pulse laser such as a titanium-sapphire laser. - Light emitted from the
light source 101 travels through theoptical fiber 102 and is collimated by thecollimator 103, thereby being output as thecollimated beam 104. - The collimated
beam 104 is split into the signal beam 107 (a beam traveling toward the object 124) and the reference beam 106 (a beam traveling toward the reference mirrors 111) by thebeam splitter 105. - The
signal beam 107 is moved by the scanningoptical system 108 in one-dimensional and two-dimensional manners. A galvanoscanner can be employed as the scanningoptical system 108. The scanningoptical system 108 performs one-dimensional scanning for B-scan of the eyeground, and two-dimensional scanning for C-scan and acquisition of a three-dimensional image. - The
signal beam 107 moved as aforementioned is adjusted by the eyepiece members 109-1 and 109-2 in accordance with the refraction of theobject 124, i.e., the eye, and is applied to theobject 124. - For example, the effect of refraction can be adjusted by moving the eyepiece member 109-2 along the optical axis.
- The
signal beam 107 applied to the eye is reflected and scattered by layers in the retina and returns along substantially the same path to thebeam splitter 105. - Meanwhile, the
reference beam 106 is reflected by the reference mirrors 111-1 and 111-2 and returns to thebeam splitter 105. - The reference mirror 111-2 is controllable with the
reference mirror stage 112. Thereference mirror stage 112 moves the reference mirror 111-2 in the optical-axis direction, whereby the length of the optical path of the reference beam 106 (hereinafter referred to as the reference-path length) is increased and decreased. - In the TD-OCT apparatus of the first embodiment, the
OCT processing unit 116, which is a computer system, controls the entire operation of the apparatus, including the scanning operation of the scanningoptical system 108 and the operations of other components such as the focusingstage 110 and thereference mirror stage 112. - The
signal beam 107 and thereference beam 106 that have returned to thebeam splitter 105 interfere with each other, are transmitted through thecollimator 114, and are converted into an electrical signal by thephotodetector 115. - In the TD-OCT apparatus of the first embodiment, a signal from a specific depth in the
object 124 can be solely detected by causing only rays of thesignal beam 107 that have traveled the same length as the reference-path length defined by the position of the reference mirror 111-2 to interfere with thereference beam 106. - The behavior of the signal beam in the eye will now be described.
-
FIGS. 5A and 5B are diagrams for describing the optical path in an exemplary TD-OCT apparatus according to the first embodiment in a case where the apparatus is used for eye measurement. -
FIG. 5A shows the optical path of asignal beam 507 in aneye 524. - The
signal beam 507 is applied through acornea 525 to the interior of theeye 524. - The
signal beam 507, which is a substantially collimated beam, is focused on aretina 526 because of the effect of refraction caused by thecornea 525 and so forth. - The
signal beam 507 is reflected and scattered by layers in theretina 526 of theeye 524, returns along the same path, and is detected as an interference signal. - With a reference mirror being at a specific position, only the
signal beam 507 from a specific layer interferes with a reference beam. Therefore, signals from different layers are detected as different signals. -
FIG. 5B shows the optical path of thesignal beam 507 in theeye 524 in a case where scanning is performed by a scanningoptical system 508. - If the scanning
optical system 508 is moved, the incident angle of thesignal beam 507 at thecornea 525 changes, whereby the focal position on theretina 526 changes. - If the reference mirror and the scanning
optical system 508 are moved during detection of the interference beam, a B-scan image of theretina 526 is acquired. - If the scanning
optical system 508 is moved with the reference mirror being fixed at a specific position during detection of the interference beam, a C-scan image, i.e., an image of a transverse section, defined by rays of thesignal beam 507 of a constant length is acquired. - The aforementioned steps of acquiring a tomogram with the TD-OCT apparatus according to the first embodiment will now be described.
- First, in step (1), a two-dimensional image extending in the optical-axis direction in which the light enters the object is acquired.
- Specifically, the scanning
optical system 108 is moved in a one-dimensional manner while the reference mirror 111-2 is controlled, whereby a B-scan image, i.e., a longitudinal sectional image, of the eyeground of theobject 124 is acquired. - The B-scan image is treated as a first preliminary image. The B-scan image extends in, for example, the X-axis direction in
FIG. 5B . - In step (2), another two-dimensional image that is substantially orthogonal to the two-dimensional image acquired in step (1) is acquired.
- Specifically, the scanning
optical system 108 is moved in a one-dimensional manner in a direction substantially orthogonal to the direction in step (1) while the reference mirror 111-2 is controlled as in step (1), whereby another B-scan image orthogonal to the first preliminary image is acquired as the second preliminary image. - The second preliminary image is a B-scan image extending in the Y-axis direction in
FIG. 5B . - In step (3), depth-position information on a desired layer is calculated from the first preliminary image acquired in step (1) and the second preliminary image acquired in step (2), i.e., longitudinal sectional information. The calculation is performed as follows.
- Three-dimensional position information on a desired layer is calculated from the first and second preliminary images.
-
FIG. 6A shows anexemplary region 626 containing a desired layer. - An X-axis
sectional image 627 is acquired as a first preliminary image. The firstpreliminary image 627 contains aportion 629 of the desired layer. - Subsequently, a Y-axis
sectional image 628 is acquired as a second preliminary image. Similar to the firstpreliminary image 627, the secondpreliminary image 628 contains aportion 630 of the desired layer. - Position information on the
portion 629 is extracted from the firstpreliminary image 627 by image processing. Likewise, position information on theportion 630 is extracted from the secondpreliminary image 628 by image processing. - With reference to the position information, a three-
dimensional surface 631, shown inFIG. 6C , containing theportions - A specific exemplary method of estimating the three-
dimensional surface 631 will now be described. - The position information on the
portion 629 is extracted from the firstpreliminary image 627 and is fitted to an appropriate function. - For example, the position information is fitted to a function of Z=X2, with the X-axis-direction center being defined as the origin.
- Likewise, the position information on the
portion 630 is extracted from the secondpreliminary image 628 and is fitted to a function of Z=Y2. Combining the two functions results in a function of Z=X2+Y2, which expresses positional information on the three-dimensional surface 631. - In step (4), the light entering the object is moved while the reference mirror is controlled in accordance with the depth-position information, whereby a three-dimensional surface image of the desired layer is acquired as follows.
- In accordance with the position information on the three-dimensional surface, i.e., the depth-position information calculated as above, the reference mirror 111-2 and the scanning
optical system 108 are controlled, whereby a C-scan image containing the three-dimensional surface 631 is acquired. -
FIG. 6D is a flowchart of a process for acquiring an image through the steps described above using the TD-OCT apparatus of the first embodiment. - The reference mirror 111-2 can be controlled by, as shown in
FIG. 1 , thereference mirror stage 112 for the purposes of both adjustment of the reference-path length when a B-scan image is acquired and tracking of the three-dimensional surface 631. - The
reference mirror stage 112, i.e., a reference-path-length adjuster, can include a stage capable of coarse adjustment performed in acquiring position information and a stage capable of fine adjustment performed in acquiring a three-dimensional surface image. -
FIG. 7 shows an exemplary reference-path-length adjuster. The adjuster includes astage 712 intended for acquisition of a B-scan image and being movable within a wide range, astage 732 provided on thestage 712 and capable of fine and quick adjustment, and a reference mirror 711-2 provided on thestage 732. In this case, it is desirable that thestage 712 be configured as a linear motor stage or the like, and thestage 732 be configured as a voice coil motor stage or the like that is movable at a high speed. - In the first embodiment, a three-dimensional surface image can alternatively be acquired by, instead of controlling the reference-path length determined by the reference mirror, controlling the point in the object at which the measuring beam guided to the object is incident.
- Control of the plane along which information is to be acquired can be performed by, other than by controlling the reference mirror, controlling the position of the entrance pupil. This control method will now be described with reference to
FIG. 8 . - Referring to
FIG. 8 , if abeam 833 entering aneye 824 through the center of the cornea is moved by a scanning optical system, an image of aplane 835 containing the line a-a′ that is parallel to the retina is acquired. - In contrast, if a
beam 834 that is parallel to thebeam 833 but does not pass through the center of the cornea is moved by the scanning optical system, an image of aplane 836 containing the line b-b′ is acquired. - Thus, by changing the position of the pupil through which the signal beam passes, the inclination of a plane along which information is to be acquired can be controlled.
- By controlling the position of the pupil in accordance with the three-
dimensional surface 631, the plane along which information is to be acquired can be changed successively, whereby a C-scan image representing the three-dimensional surface 631 can be acquired. - The position of the pupil can be controlled by controlling the position of an object to be examined. Alternatively, the position of the pupil may be controlled by controlling the position optically conjugate to the pupil of the object, or by controlling the position of the collimator intended for detection.
- While the first embodiment concerns a TD-OCT apparatus, the present invention is not limited thereto.
- For example, the present invention may also be applied to a case where a tomogram is acquired by using a frequency-domain OCT (FD-OCT) apparatus that does not require mechanical scanning.
- An optical tomographic imaging method according to a second embodiment of the present invention will now be described in which a TD-OCT apparatus is used.
- According to another aspect of the present invention, an optical tomographic imaging method according to a second embodiment includes the following steps (1) to (4): (1) acquiring, as a first preliminary image, a two-dimensional image of an object to be examined, the image extending in an optical-axis direction in which light enters the object; (2) acquiring, as second and subsequent preliminary images, a plurality of two-dimensional images of the object, the images extending in the optical-axis direction and being substantially parallel to the two-dimensional image acquired in step (1); (3) calculating depth-position information on a desired layer from the image acquired in step (1) and the images acquired in step (2); and (4) acquiring a surface image of the desired layer by moving the light entering the object while controlling a reference mirror in accordance with the depth-position information.
- The second embodiment is basically the same as the first embodiment, except that, in step (2), a plurality of longitudinal sectional images substantially parallel to the first preliminary image are acquired as the second and subsequent preliminary images.
- In step (2), acquiring as many longitudinal sectional images as possible is effective for improving the accuracy in the estimation of the three-dimensional surface performed in step (3). Nevertheless, the more the number of images to be acquired, the longer the time required for acquiring such images. Therefore, the number of images to be acquired needs to be determined considering the speed of image acquisition.
- An exemplary method of calculating the three-dimensional surface according to the second embodiment will now be described.
- As in the first embodiment, position information on the portion of the desired layer is extracted from the first preliminary image and is fitted to an appropriate function, that is, a function of Z=X2 is obtained.
- Subsequently, from the second preliminary image parallel to the first preliminary image, a function of Z=X2+1 (where Y=1) is obtained.
- Likewise, from the third and fourth preliminary images, functions of Z=X2+2 (where Y=2) and Z=X2+3 (where Y=3) are obtained, respectively.
- Considering the foregoing functions, it is understood that position information representing the surface of the desired layer can be expressed as Z=X2+Y.
- Moreover, if images that are substantially orthogonal to the first preliminary image and images that are substantially parallel to the first preliminary image are both acquired in step (2), the accuracy in the three-dimensional position information is further improved.
- An optical tomographic imaging method according to a third embodiment of the present invention will now be described in which a TD-OCT apparatus utilizing a plurality of pieces of one-dimensional information is used.
- According to an aspect of the present invention, an optical tomographic imaging method according to a third embodiment includes the following steps (1) to (3): (1) acquiring, as a first preliminary image, a plurality of one-dimensional images of an object to be examined, the images extending in an optical-axis direction in which light enters the object; (2) calculating depth-position information on a desired layer from the images acquired in step (1); and (3) acquiring a surface image of the desired layer by moving the light entering the object while controlling a reference mirror in accordance with the depth-position information.
- In step (1), a plurality of A-scan images (pieces of one-dimensional information) are acquired. In step (2), position information on the desired layer is calculated from the plurality of A-scan images.
- In the third embodiment, acquisition of the A-scan images in step (1) does not require mechanical scanning. Therefore, an FD-OCT apparatus capable of high-speed acquisition of an A-scan image is desirably used.
- Steps (2) and (3) are the same as steps (3) and (4) in the first embodiment.
- Examples of the present invention will now be described.
- Example 1 will first be described in which the TD-OCT apparatus according to any of the embodiments of the present invention is applied to eyeground examination. In Example 1, a surface image of the pigmented layer in the retina is acquired. The present invention is not limited to such eyeground examination.
-
FIG. 9 is a diagram for describing a TD-OCT apparatus and steps of a tomographic imaging method in Example 1. - In Example 1, light emitted from a
light source 901 is guided by a single-modeoptical fiber 902. The light output from the end of theoptical fiber 902 is collimated into acollimated beam 904 by acollimator 903. The collimatedbeam 904 is split into areference beam 906 and asignal beam 907 by abeam splitter 905. - The
light source 901 is an SLD having a center wavelength of 840 nm and a wavelength width of about 50 nm. - The
reference beam 906 is shifted by afrequency shifter 937 such that the optical frequency thereof is shifted by Δf, is subsequently reflected by reference mirrors 911-1 and 911-2, and is further directed by a reflecting mirror 911-3 toward a combiningoptical system 938. - The positions of the reference mirrors 911-1 and 911-2 are controlled by
mirror position adjusters - The
mirror position adjuster 912 is a linear motor stage movable with a stroke of 25 mm. Themirror position adjuster 932 is a voice coil motor stage movable with a stroke of 1 mm. - The
signal beam 907 first enters a beam guide-splitter 940 and is guided into an examination optical system facing aneye 924, i.e., an object to be examined. - The examination optical system includes the beam guide-
splitter 940, a scanningoptical system 908, a scanning lens 909-1, and an eyepiece lens 909-2. - The eyepiece lens 909-2 is moved in the optical-axis direction of the beam incident thereon by a
focus position adjuster 910. The scanningoptical system 908 tilts the chief ray of thesignal beam 907 in two directions orthogonal to the optical axis. - Thus, the beam transmitted through the scanning lens 909-1 and the eyepiece lens 909-2 is moved so as to have a variable angle with respect to the pupil (iris) of the
eye 924. - Consequently, because of the optical effect of the
eye 924, an eyeground-examination target site 941 is scanned within a plane (XY plane) on the eyeground, the plane being perpendicular to the optical axis extending in the depth direction. - Some rays of reflections and backscattered light from the
target site 941 return along substantially the same path as the incident path to thetarget site 941. Such return rays enter the beam guide-splitter 940, and some rays thereof are further guided to the combiningoptical system 938. - The combining
optical system 938 combines thesignal beam 907 and thereference beam 906 together and then splits the combined beam into interference beams 939-1 and 939-2 having composite amplitudes, i.e., complex amplitudes, obtained by adding thesignal beam 907 and thereference beam 906, the composite amplitudes being out of phase with each other. Some rays of the interference beams 939-1 and 939-2 enter condenser optical systems 914-1 and 914-2, respectively. - The rays of the interference beams 939-1 and 939-2 are optically coupled to respective single-mode optical fibers connecting the condenser optical systems 914-1 and 914-2 to a
photoelectric conversion detector 915, whereby components of the rays matching the modes of the respective fibers are selected, propagate inside the fibers, and enter thephotoelectric conversion detector 915. - The
photoelectric conversion detector 915 converts the rays into electrical signals. The electrical signals are transmitted to anOCT processing unit 916. - Here, the components matching the modes of the fibers each refer to a confocal component that is conjugate to a ray scattered from a point on the
target site 941. - The composite amplitudes, i.e., the complex amplitudes, of the interference beams 939-1 and 939-2 obtained by adding the
reference beam 906 and thesignal beam 907 are each an amplitude of heterodyne interference having a carrier frequency corresponding to a frequency difference Δf produced by thefrequency shifter 937. - The wave of the heterodyne interference is an oscillation having a frequency Δf, and the absolute amplitude of the oscillation changes temporally.
- The composite amplitudes of the interference beams 939-1 and 939-2 received by the respective condenser optical systems 914-1 and 914-2 have temporal oscillations that are in phase opposition. Thus, by the a TD-OCT method, C-scan images (images each extending within an XY plane intersecting the Z-axis direction, i.e., the depth direction) at focal positions at different depths in the object can be acquired.
- The process of acquiring a surface image by the optical tomographic imaging method in Example 1 will now be described.
-
FIGS. 10A to 10C are diagrams for describing steps of estimating a three-dimensional surface in Example 1. - First, a B-scan image of a
region 1026 containing a desired layer is acquired. - Specifically, while the
mirror position adjuster 912 is moved, the scanningoptical system 908 is controlled such that scanning is performed in the X-axis direction inFIG. 10A , whereby a first B-scan image 1027 is acquired. - Subsequently, while the
mirror position adjuster 912 is moved, the scanningoptical system 908 is controlled such that scanning is performed in the Y-axis direction inFIG. 10A , whereby a second B-scan image 1028 representing a longitudinal section orthogonal to the first B-scan image 1027 is acquired. - The first B-
scan image 1027 represents a longitudinal section of the retina and contains aportion 1029 of the pigmented layer, i.e., the desired layer. - Likewise, the second B-
scan image 1028 contains aportion 1030 of the pigmented layer, i.e., the desired layer. - The two-dimensional shape of the pigmented layer in the
portion 1029 is calculated from the first B-scan image 1027 by image processing. - In general, the pigmented layer has the highest reflectivity in the retina. Therefore, the shape of the pigmented layer can be calculated by, for example, extracting regions having high brightness with respect to the Z-axis direction.
- Likewise, the two-dimensional shape of the pigmented layer in the
portion 1030 is calculated from the second B-scan image 1028. Subsequently, three-dimensional position information on the pigmented layer is estimated from the calculated two-dimensional shapes that are orthogonal to each other, whereby asurface 1031 representing the pigmented layer is acquired. - Then, while the
mirror position adjuster 932, which is a fine-motion stage, is controlled, the scanningoptical system 908 is moved such that interference occurs along thesurface 1031, whereby a C-scan image of thesurface 1031 is acquired. The C-scan image in its entirety represents the surface of the pigmented layer. - Example 2, different from Example 1, will now be described in which the TD-OCT apparatus according to any of the embodiments of the present invention is applied to eyeground examination. In Example 2, a surface image of the pigmented layer in retinal cells of an eye to be examined is acquired.
- Example 2 is basically the same as Example 1, except that the B-scan images acquired for calculation of three-dimensional information on the pigmented layer are substantially parallel to each other.
- In Example 2, a tomogram of the retina is acquired by using the apparatus shown in
FIG. 9 , as in Example 1. - The process of acquiring a tomogram of the retina will now be described.
-
FIGS. 11A to 11C are diagrams for describing steps of estimating a three-dimensional surface in Example 2. - In Example 2, a B-scan image of a
region 1126 containing a desired layer is first acquired. - Specifically, while the
mirror position adjuster 912 is moved, the scanningoptical system 908 is controlled such that scanning is performed in the X-axis direction inFIG. 11A , whereby a first B-scan image 1127 is acquired. - Subsequently, the scanning
optical system 908 is moved in the Y-axis direction inFIG. 11A and is fixed at a specific position. Then, while themirror position adjuster 912 is moved, the scanningoptical system 908 is controlled such that scanning is performed in the X-axis direction inFIG. 11A , whereby a second B-scan image 1142 parallel to the first B-scan image 1127 is acquired. - Likewise, the scanning
optical system 908 is reversely moved in the Y-axis direction inFIG. 11A and is fixed at a specific position. Then, while themirror position adjuster 912 is moved, the scanningoptical system 908 is controlled such that scanning is performed in the X-axis direction inFIG. 11A , whereby a third B-scan image 1143 parallel to the first B-scan image 1127 is acquired. - Thus, three B-scan images that are substantially parallel to each other are acquired.
- The three B-
scan images respective portions - Respective pieces of position information on the
portions - From the extracted pieces of position information, three-dimensional position information on the pigmented layer, i.e., a
surface 1131, is estimated. - In accordance with the estimated position information, a C-scan image of the
surface 1131 is acquired, as in Example 1. - Example 3, different from Examples 1 and 2, will now be described in which the TD-OCT apparatus according to any of the embodiments of the present invention is applied to eyeground examination. In Example 3, a surface image of the pigmented layer in retinal cells of an eye to be examined is acquired.
- In Example 3, three-dimensional information on the pigmented layer is calculated from a plurality of A-scan images.
- Such a method can be performed in the same manner as in Example 1. Alternatively, an FD-OCT apparatus capable of high-speed acquisition of an A-scan image may be used.
- In Example 3, a tomogram of the retina is acquired by using the TD-OCT apparatus shown in
FIG. 9 , as in Example 1. - The process of acquiring a tomogram of the retina will now be described.
-
FIGS. 12A to 12C are diagrams for describing steps of estimating a three-dimensional surface in Example 3. In Example 3, a plurality of sampling points are first designated in aregion 1226 containing a desired layer. - The sampling points are designated randomly. For example, nine points 1246-1 to 1246-9 shown in
FIG. 12A that are evenly arranged in the X- and Y-axis directions can be designated. - The scanning
optical system 908 is controlled such that thesignal beam 907 is incident at one of the designated sampling points 1246-1 to 1246-9, for example, the point 1246-1. - The scanning
optical system 908 is fixed at a position corresponding to the point 1246-1, and themirror position adjuster 912 is moved, whereby an A-scan image at the point 1246-1 is acquired. - In Example 3, the A-scan image is acquired by moving the
mirror position adjuster 912. If an FD-OCT apparatus is used, there is no need to adjust the reference-path length, and information on an A-scan image can be acquired collectively. - Next, the scanning
optical system 908 is fixed at another position corresponding to, for example, the point 1246-2, and themirror position adjuster 912 is moved, whereby an A-scan image at the point 1246-2 is acquired. - Likewise, A-scan images at the points 1246-3 to 1246-9 are acquired.
FIG. 12B shows examples of the acquired A-scan images. - The A-scan images each contain a signal representing a
portion 1230 of the pigmented layer. Therefore, pieces of position information on therespective portions 1230 are extracted by image processing. - From the positions of the
portions 1230 at the sampling points 1246-1 to 1246-9, three-dimensional position information on the pigmented layer, i.e., asurface 1231, is estimated. In accordance with the estimated position information, a C-scan image representing thesurface 1231 is acquired, as in Example 1. - Example 4, different from Examples 1 to 3, will now be described in which the TD-OCT apparatus according to any of the embodiments of the present invention is applied to eyeground examination. In Example 4, a surface image of the pigmented layer in retinal cells of an eye to be examined is acquired.
- Example 4 is basically the same as Example 1, except that the plane along which information is to be acquired is controlled by controlling the position of the entrance pupil.
-
FIG. 13 is a diagram for describing a TD-OCT apparatus and steps of a tomographic imaging method in Example 4. - In Example 4, a tomogram of the retina is acquired by using the apparatus shown in
FIG. 13 in the following manner. - First, light emitted from a
light source 1301 is guided through a single-mode optical fiber 1302-1, and is split into a signal beam and a reference beam by afiber coupler 1348. Thelight source 1301 is an SLD having a center wavelength of 840 nm and a wavelength width of about 50 nm. The signal beam resulting from the splitting by thefiber coupler 1348 is guided through a single-mode optical fiber 1302-2 to afiber coupler 1349 and further to a collimator 1314-1, and is converted into acollimated signal beam 1307 by the collimator 1314-1. - The reference beam is guided through a single-mode optical fiber 1302-3 to a
collimator 1351 and is output as acollimated reference beam 1306 from thecollimator 1351. - The
reference beam 1306 is shifted by afrequency shifter 1337 such that the optical frequency thereof is shifted by Δf, is subsequently reflected by reference mirrors 1311-1 and 1311-2, and is further directed by a reflecting mirror 1311-3 toward a collimator 1314-2. - The positions of the reference mirrors 1311-1 and 1311-2 are controlled by a
mirror position adjuster 1312 such that the reference-path length becomes a specific value. - The
signal beam 1307 is guided into an examination optical system facing aneye 1324, i.e., an object to be examined. The examination optical system includes a beam guide-splitter, a scanningoptical system 1308, a scanning lens 1309-1, and an eyepiece lens 1309-2. - The eyepiece lens 1309-2 is moved in the optical-axis direction of the beam incident thereon by a
focus position adjuster 1310. - The scanning
optical system 1308 tilts the chief ray of thesignal beam 1307 in two directions orthogonal to the optical axis. - Thus, the beam transmitted through the scanning lens 1309-1 and the eyepiece lens 1309-2 is moved so as to have a variable angle with respect to the pupil (iris) of the
eye 1324. - Consequently, because of the optical effect of the
eye 1324, an eyeground-examination target site 1341 is scanned within a plane (XY plane) on the eyeground, the plane being perpendicular to the optical axis extending in the depth direction. - Some rays of reflections and backscattered light from the
target site 1341 return along substantially the same path as the incident path to thetarget site 1341. Such return rays enter the beam guide-splitter, and some rays thereof are further guided to the collimator 1314-1. Example 4 differs from Example 1 in including apupil position adjuster 1347 that adjusts the position of the pupil of the collimator 1314-1. - The
pupil position adjuster 1347 may be a linear motor stage or the like, or a voice coil motor stage that is movable at a higher speed. - The inclination of the plane along which the eyeground is to be scanned can be controlled, as shown in
FIG. 8 , by horizontally moving thepupil position adjuster 1347. - The
signal beam 1307 condensed by the collimator 1314-1 and thereference beam 1306 condensed by the collimator 1314-2 are combined together by afiber coupler 1350. The resulting interference beams are converted into electrical signals by aphotoelectric conversion detector 1315. - The electrical signals are transmitted to an
OCT processing unit 1316. - The composite amplitudes, i.e., the complex amplitudes, of the interference beams obtained by adding the
reference beam 1306 and thesignal beam 1307 are each an amplitude of heterodyne interference having a carrier frequency corresponding to a frequency difference Δf produced by thefrequency shifter 1337. - The wave of the heterodyne interference is an oscillation having a frequency Δf, and the absolute amplitude of the oscillation changes temporally.
- The composite amplitudes of the interference beams have temporal oscillations that are in phase opposition. Thus, by the a TD-OCT method, C-scan images (images each extending within an XY plane intersecting the Z-axis direction, i.e., the depth direction) at focal positions at different depths in the object can be acquired.
- The process of acquiring a surface image is the same as in Example 1.
- In the same manner as in Example 1, first and second B-scan images are acquired. From the first and second B-scan images, information on the three-dimensional surface representing the pigmented layer is estimated.
- In accordance with the estimated information on the three-dimensional surface, the scanning
optical system 1308 is moved while thepupil position adjuster 1347 is controlled, whereby a C-scan image of thesurface 1031 shown inFIG. 10C is acquired. - The plane along which measurement is to be performed is changed by controlling the
pupil position adjuster 1347 in accordance with the inclination at each measurement point on thesurface 1031. Thus, images composing thecurved surface 1031 are successively acquired. - If the
mirror position adjuster 1312 is also controlled during the above operation, the desired surface can be measured more precisely. - Example 5 will now be described.
- Example 5 concerns a case where an object having a simple three-dimensional shape, such as a pipe-like shape, is measured. In this case, position information on the desired layer can be estimated with a single B-scan image.
- A tomogram of the retina is acquired by using the apparatus shown in
FIG. 9 , as in Example 1. - The process of acquiring a tomogram of the retina will now be described.
-
FIGS. 14A to 14C are diagrams for describing steps of estimating a three-dimensional surface in Example 5. - First, a B-scan image of a
region 1426 containing a desired layer is acquired. - Specifically, while the
mirror position adjuster 912 is moved, the scanningoptical system 908 is controlled such that scanning is performed in the X-axis direction inFIG. 14A , whereby a first B-scan image 1427 is acquired. - The first B-
scan image 1427 contains aportion 1430 of the desired layer. Position information on theportion 1430 is extracted by image processing. - In Example 5, the depth position does not change with the change in the Y-axis-direction position. Therefore, only with the position information on the
portion 1430, three-dimensional position information on asurface 1431 can be calculated. - For example, if the
portion 1430 is fitted to a function of Z=X2, thesurface 1431 is also expressed as Z=X2. - In accordance with the position information acquired in this manner, a C-scan image of the
surface 1431 is acquired, as in Example 1. - While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications and equivalent structures and functions.
Claims (8)
1. An ophthalmologic apparatus for acquiring a tomographic image of a fundus of an eye to be examined based on a light obtained by combining a return light from the eye to be examined irradiated with a measuring light and a reference light corresponding to the measuring light, the ophthalmologic apparatus comprising:
a scanning unit configured to scan the measuring light on the fundus of the eye to be examined; and
a calculation unit configured to calculate three-dimensional position information of a predetermined layer of the fundus based on a plurality of pieces of longitudinal sectional information of the fundus.
2. The ophthalmologic apparatus according to claim 1 , further comprising a control unit configured to control the scanning unit so as to obtain a plurality of two-dimensional tomographic images which intersect with each other as the plurality of pieces of longitudinal sectional information,
wherein the calculation unit calculates the position information based on the plurality of two-dimensional tomographic images.
3. The ophthalmologic apparatus according to claim 1 , further comprising a control unit configured to control the scanning unit so as to obtain a plurality of two-dimensional tomographic images which are substantially parallel to each other as the plurality of pieces of longitudinal sectional information,
wherein the calculation unit calculates the position information based on the plurality of two-dimensional tomographic images.
4. The ophthalmologic apparatus according to claim 1 , further comprising a control unit configured to control the scanning unit so as to obtain a plurality of pieces of one-dimensional information in a depth direction of the fundus as the plurality of pieces of longitudinal sectional information,
wherein the calculation unit calculates the position information based on the plurality of pieces of one-dimensional information.
5. The ophthalmologic apparatus according to claim 1 , wherein the calculation unit calculates a three-dimensional curved surface of the predetermined layer based on the plurality of pieces of longitudinal sectional information and calculates the position information based on the three-dimensional curved surface.
6. The ophthalmologic apparatus according to claim 1 , further comprising:
an optical-path length changing unit configured to change an optical path of the reference light; and
a control unit configured to control the optical-path length changing unit in accordance with the position information and to obtain a three-dimensional surface image of the fundus
7. An optical tomographic imaging apparatus for acquiring a tomographic image of an examination object based on a light obtained by combining a return light from the examination object irradiated with a measuring light and a reference light corresponding to the measuring light, the optical tomographic imaging apparatus comprising:
a scanning unit configured to scan the measuring light on the fundus of the examination object; and
a calculation unit configured to calculate three-dimensional position information of a predetermined portion of the examination object based on a plurality of pieces of longitudinal sectional information of the examination object.
8. The optical tomographic imaging apparatus according to claim 7 , further comprising a control unit configured to control the scanning unit so as to obtain a plurality of two-dimensional tomographic images which intersect with each other as the plurality of pieces of longitudinal sectional information,
wherein the calculation unit calculates the position information based on the plurality of two-dimensional tomographic images.
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US9119571B2 (en) | 2011-02-04 | 2015-09-01 | Tomey Corporation | Ophthalmologic apparatus |
US9999349B2 (en) | 2011-02-04 | 2018-06-19 | Tomey Corporation | Ophthalmologic apparatus for measuring position of measuring portion inside eye |
US20130003075A1 (en) * | 2011-06-30 | 2013-01-03 | Canon Kabushiki Kaisha | Imaging apparatus |
US9456742B2 (en) | 2011-10-27 | 2016-10-04 | Canon Kabushiki Kaisha | Ophthalmologic apparatus |
Also Published As
Publication number | Publication date |
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JP5679630B2 (en) | 2015-03-04 |
JP2010151713A (en) | 2010-07-08 |
US20100165289A1 (en) | 2010-07-01 |
US7954948B2 (en) | 2011-06-07 |
US8317326B2 (en) | 2012-11-27 |
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